Waves and space · GCSE Physics
Lenses
Teacher-written GCSE Physics revision on converging and diverging lenses: ray diagrams, focal length, magnification, the eye, and correcting long sight and short sight.
Converging lens: parallel rays meet at the principal focus. Real image when the object is beyond F — inverted on a screen. Diverging lens: rays spread as if from a virtual focus.
The important bits
What you need to know
- 1
A lens is a shaped piece of transparent material (usually glass or plastic) that refracts light. A converging (convex) lens is thicker in the middle; a diverging (concave) lens is thinner in the middle.
- 2
The principal axis is a line through the centre of the lens at right angles to it. Parallel rays hitting a converging lens refract and meet at the principal focus, F, on the far side.
- 3
Focal length f is the distance from the lens to F. Shorter f means a stronger lens (more bending). Real images can be projected on a screen; virtual images cannot — they are seen by the eye looking into the lens.
- 4
For a converging lens with the object beyond F: a real, inverted image forms on the other side. A camera and the eye use this arrangement. Object at 2F gives image same size at 2F; closer than F gives a virtual, upright magnified image (magnifying glass).
- 5
Magnification = image height ÷ object height (no units). A value greater than 1 means enlarged; less than 1 means diminished. image distance and object distance follow the lens equation on Higher tier routes.
- 6
The eye: the cornea and lens focus light onto the retina. Ciliary muscles change lens shape for accommodation (near and far). Long sight (hyperopia): eye too short or lens too weak — image forms behind retina; corrected with a converging spectacle lens.
- 7
Short sight (myopia): eye too long or lens too strong — image forms in front of retina; corrected with a diverging spectacle lens so rays diverge before entering the eye.
- 8
Ray diagrams must show two (or three) construction rays from the object tip: parallel to axis → through F; through centre → undeviated; through F → emerges parallel (for converging lens). Label object, image, F and the lens.
Quotations worth analysing
Short evidence. Real method.
“Parallel rays of light are brought to a focus by a converging lens.”
Draw the ray parallel to the principal axis, refracted through F. Without F labelled, the diagram is incomplete.
“Magnification = image height / object height”
Use the same units (often cm on a diagram). Inverted image: height is still positive for ratio unless the question defines sign.
“A diverging lens always produces a virtual, upright image smaller than the object.”
Short-sight correction relies on this: rays appear to come from a nearer virtual focus so the eye’s lens can focus them on the retina.
Go deeper
Ray diagrams that actually score
Pick the object tip. Ray 1: parallel to axis, refract through F on the far side (converging). Ray 2: straight through the optical centre — no deviation. Where rays cross is the image tip. For an object beyond 2F, image is between F and 2F, smaller, inverted, real — camera mode. Between F and 2F: image beyond 2F, magnified, inverted, real — projector mode. Inside F: rays diverge after the lens; trace them back with dotted lines to a virtual upright magnified image on the same side as the object — magnifying glass. Examiners penalise missing arrowheads, missing F, or rays that do not obey the rules. Scale does not need to be artistic; geometry must be correct.
Go deeper
The eye and spectacle lenses
The eye is a converging system. Light from a distant object should focus exactly on the retina. Long-sighted people see distant objects more clearly than near ones because the eyeball is too short or the lens too weak — near rays would focus behind the retina. A converging spectacle lens adds extra convergence before light enters the eye. Short-sighted people see near objects more clearly; distant rays focus in front of the retina because the eyeball is too long or the lens too strong. A diverging lens spreads rays so the eye’s own lens can bring them to focus on the retina. Do not say “the lens of the glasses focuses the image on the retina” without naming long versus short sight and converging versus diverging.
Go deeper
Real versus virtual — one sentence each
Real: rays actually meet; can form on a screen; inverted for a single converging lens with object outside F. Virtual: rays only appear to come from a point; cannot be projected; upright in the standard magnifying-glass case. A plane mirror also gives a virtual image — lenses reuse the same vocabulary. Magnification links to astronomy and microscopes: an objective forms a real image that an eyepiece may treat as an object for further magnification on Triple routes. At GCSE Combined level, perfect the single converging lens ray diagram and the two eye defects.
See the idea in action
An object 3.0 cm tall is placed 30 cm from a converging lens of focal length 10 cm (object beyond 2F if 2F = 20 cm). Ray diagram shows a real inverted image between F and 2F on the other side, height about 1.5 cm. Magnification = 1.5 / 3.0 = 0.5 — diminished. If the object is moved to 5 cm (inside F), the image is virtual, upright, magnified on the same side as the object.
Exam technique
Turn knowledge into marks
Label F, 2F and the lens on every diagram. State real or virtual, upright or inverted, and magnified or diminished in words after the drawing.
Common mistakes
Do not give these marks away
- 01
Drawing rays that do not pass through F for a converging lens, or forgetting arrow direction.
- 02
Prescribing the wrong spectacle lens: converging for long sight, diverging for short sight — not the reverse.
- 03
Calling any image in a lens “virtual” without checking whether rays actually converge.
Which lens is used to correct short-sightedness?
AA converging lens, because the eye focuses rays too weakly
BA diverging lens, because the eye focuses distant rays too strongly in front of the retina
CA plane mirror
DNo lens — only laser surgery is possible at GCSE
Show the answer
A diverging lens, because the eye focuses distant rays too strongly in front of the retina. Short sight needs rays to spread before entering the eye. A diverging lens produces virtual images and is the standard correction.
Quick questions
If this is the bit you searched
What is the focal length?
The distance from the lens to the principal focus F, where parallel rays meet (converging lens) or appear to come from (diverging lens).
When does a converging lens give a real image?
When the object is outside the focal point F on a converging lens — rays converge to a real inverted image that could be shown on a screen.
What is magnification?
Image height divided by object height. Greater than 1 means enlarged; less than 1 means diminished.
How is long sight corrected?
With a converging spectacle lens that adds convergence so near objects focus on the retina instead of behind it.